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Income resulting from oil and natural gas producing activites (Details 3) (USD $) | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Revenues: |  |  |  |  |  |
Sales to affiliates | $ 45,099,000 |  | $ 35,966,000 |  | $ 93,713,000 |
Sales to external customers | 497,018,000 |  | 379,647,000 |  | 348,428,000 |
Realized gain on commodity derivatives | 173,000 |  | 33,628,000 |  | 9,618,000 |
Unrealized (gain) loss on commodity derivatives | (6,267,000) |  | (624,000) |  | 1,827,000 |
Production costs | 144,136,000 |  | 134,795,000 |  | 140,606,000 |
Depreciation, depletion and amortization | 182,352,000 | [1] | 157,078,000 | [1] | 139,539,000 | [1]
Write-downs of oil and natural gas properties | 0 |  | 391,800,000 |  | 0 |
Pretax income (loss) | 209,535,000 |  | (235,056,000) |  | 173,441,000 |
Income tax expense (benefit) | 75,836,000 |  | (88,612,000) |  | 63,655,000 |
Results of operations for producing activities | 133,699,000 |  | (146,444,000) |  | 109,786,000 |
Accretion of discount for asset retirement obligations oil and gas | $ 3,600,000 |  | $ 3,300,000 |  | $ 3,600,000 |
[1] | Includes accretion of discount for asset retirement obligations of $3.6 million, $3.3 million and $3.6 million for the years ended December 31, 2013, 2012 and 2011, respectively, as discussed in Note 10.

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- Definition Depreciation, depletion and amortization, and valuation provisions (impairments) related to oil and gas producing activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 23 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62136-109447 + Details
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- Definition Costs related to the impairment of oil and gas properties. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-10.(c)(3)(ii)(A)(1)) -URI http://asc.fasb.org/extlink&oid=27015464&loc=d3e511914-122862 + Details
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- Definition The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of open agreements to purchase or sell mineral resources, energy, and agricultural products at some future point held at each balance sheet date, that was included in earnings for the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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